Alternatively referred to as autorecover, autosave is a feature that automatically saves the document you are editing in case the computer freezes or the power goes out. By default, many programs will autosave every 5 to 10 minutes if changes have been made in the file. The frequency of how often the program automatically saves can often be adjusted through the programs option menu.
If the computer happens to be shut off without the file being saved manually, when opening the program it should open the last autosave to help prevent all your work from being lost. However, any changes that were made after the autosave will be lost.